Screening methods for the rapid determination of Y chromosomal ploidy (peripheral blood leukocyte fluorescence analysis) are being applied to male participants in the national Health and Nutrition Examination Survey. Analyses are thus far completed on 2010 males from this population (plus 891 from other populations), and 2000 more will be studied in the coming year. The physical and psychosocial characteristics of individuals with the YY karyotype will be studied, by means of the other data accumulated on these subjects by the HANES. The present study will thus both determine the prevalence of YY aneuploidy in a general, ambulatory population of adult males, and point out any medical or other concomitants that may be causally related to this karyotype.